eGFR blood test: normal range explained

An eGFR is not measured directly. The lab measures creatinine, then puts it into a formula with your age and sex to estimate your glomerular filtration rate: how many millilitres of blood your kidneys filter per minute, adjusted to a standard body surface area of 1.73 square metres. MedlinePlus says eGFR is a more accurate way to judge kidney health than creatinine alone, and some labs use cystatin C instead of creatinine.

Doctors use eGFR to screen for kidney disease in people without symptoms, to help diagnose it in people with symptoms, and to follow people being treated for chronic kidney disease, often beside creatinine and a urine albumin test. The National Kidney Foundation says trends over time are often more helpful than one number, and Testing.com notes that an eGFR below 60 for three months is one of the criteria for chronic kidney disease.

Because eGFR is worked out from creatinine, anything that moves creatinine moves eGFR without the kidneys changing.

• Muscle mass: a very muscular body or a muscle-wasting condition shifts creatinine, so the estimate can be too low or too high.
• Meat, creatine supplements and hard exercise before the test raise creatinine, which makes eGFR look lower than it is; a vegetarian diet can make it look higher.
• Age: eGFR declines with age even without kidney disease, and the National Kidney Foundation publishes average values by decade.
• Pregnancy, severe liver disease, nephrotic syndrome and a past organ transplant can affect the estimate, which your doctor considers.
• Some medicines, such as cimetidine and trimethoprim, raise creatinine and so lower eGFR without harming the kidneys. Tell your doctor what you take.